The CDU (Christlich-Demokratische Union Deutschlands, Christian-Democratic Union of Germany) is the biggest conservative, centre-right party in Germany. Its sister-party in Bavaria is the CSU  the CDU is active in the rest of Germany. Both parties together form a parliamentary group in the Bundestag. The CDU grew out of several christian-democratic groups formed after World War Two based on traditions of the Zentrum (Center Party) in the Weimar Republic and other groups.

From 1949 to 1966 the CDU under chancellors Adenauer and later Erhard was the biggest party in centre-right coalitions.

From 1966 to 1969 it formed together with the SPD a 'grand coalition' (under CDU chancellor Kiesinger).

From 1969 to 1982 the CDU was in opposition to the SPD-FDP coalition government.

From 1982 to 1998 a coalition (under Chancellor Kohl) of CDU, CSU and FDP formed the federal government.

Since 1998 the CDU is once again in opposition (now under the leadership of Angela Merkel).

I now have some, albeit sketchy, information about the history of CDU flags. This seems to be a very difficult story, as they obviously change their logo and their flag every four to five years or so. The last flag had been introduced around 1998. Recently the CDU introduced a new logo together with a whole new corporate identity (CI). For details of the logos, their use, some history of the logos, see the CDU website.
The main feature of the logo is, that it always has to be the same: a red inscription 'CDU' (new font) on a white rectangle of proportions 1:3. Any additional symbols (regional symbols) or text have to be outside the white rectangle. If shown on a coloured background this logo is usually shown on an orange field.
The flag is an orange field with the white rectangle at the bottom. The colours are defined in the CI as follows: orange RGB 255/153/0; red RGB 235/39/41. Although the latter colour is not browser-safe, I chose to use the prescribed colour. I first saw the CDU flag three weeks ago in Berlin at the party headquarters. There it flows from the top of the building as well as in front of the building.Marcus Schmöger, 24 December 2003